­People spit for a variety of reasons. We've all employed the technique to remove a hair or some other distasteful object from our mouths. People who chew tobacco do it for obvious reasons. Ball players do it because they're nervous, bored or looking to showcase their masculinity. And people in many different cultures spit on their enemies to show disdain.

­Thanks to a phenomenon known as direct-to-consumer genetic testing or at-home ­genetic testing, people are spitting today for a much more productive (and perhaps more sophisticated) reason -- to get a glimpse of their own DNA.

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That's right. Your saliva contains a veritable mother lode of biological material from which your genetic blueprint can be determined. For example, a mouthful of spit contains hundreds of complex protein molecules -- what scientists call enzymes -- that aid in the digestion of food. Swirling around with those enzymes are cells sloughed off from the inside of your cheek. Inside each of those cells lies a nucleus, and inside each nucleus, chromosomes. Chromosomes themselves are made up of DNA, the now-ubiquitous shorthand for deoxyribonucleic acid, the double-stranded molecule that gets much of the credit for what we look like and how we act.

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Of course, you can't look at your own spit and see sloughed-off cells, the DNA they contain or the genetic information coded in the long chain of base pairs. You need special equipment and scientists who know how to use it. You also need trained counselors who can help you interpret the data once you get it back. That's where companies like 23andMe, deCODEme and Navigenics come in. They give you the tools, resources and infrastructure necessary to learn more about what makes you tick at a cellular level. They each do it slightly differently, and they each reveal different aspects of your DNA profile.

Let's see how these companies transform spit into state-of-the-art science.

Types of Genetic Tests

A German police officer takes a saliva sample from a young man at a primary school in northwestern Germany. The country has used voluntary genetic testing as an unprecedented evidence-gathering technique.

­Genetic tests analyze DNA present in blood and other tissue to find genetic disorders -- diseases linked to specific gene variations or mutations. About 900 such tests exist, ranging from more invasive procedures that require a trip to the hospital to the new generation of at-home tests that demand nothing more than spitting into a sterile, mini-sized spittoon. Here's a quick overview.

Prenatal testing may involve sampling and testing the DNA of a fetus. One common test under this umbrella is amniocentesis, which requires a physician to insert a needle into the water-filled sac surrounding the fetus to withdraw a small amount of fluid. In a lab, workers culture fetal cells from the amniotic fluid to obtain a sufficient quantity of DNA. Then they analyze the DNA for chromosome abnormalities that can lead to diseases or conditions such as Down s­yndrome and spina bifida.

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Another approach to genetic testing is gene sequencing, which identifies all of the building blocks, or nucleotides, of a specific gene. Once a person's gene has been sequenced, doctors can compare the gene against all known variations to see if it is normal or defective. For example, inherited alterations in the genes called BRCA1 and BRCA2 (short for "breast cancer 1" and "breast cancer 2") are associated with many cases of breast cancer.

Next up is single nucleotide polymorphism (SNP) testing. Nucleotides have three parts: a phosphate group, joined to a pentose sugar, bonded to a nitrogenous base. You can probably recite the various bases along with us -- adenine, guanine, cytosine, thymine and uracil (in RNA). Together, these nucleotides can combine in nearly infinite ways to account for all of the variation we see within and between species. Interestingly, the sequence of nucleotides in any two people is more than 99 percent identical [source: 23andMe]. Only a few nucleotides separate you from a complete stranger. These variations are called single nucleotide polymorphisms, or SNPs (pronounced "snips").

To run a SNP test, scientists embed a subject's DNA into a small silicon chip containing reference DNA from both healthy individuals and individuals with certain diseases. By analyzing how the SNPs from the subject's DNA match up with SNPs from the reference DNA, the scientists can determine if the subject might be predisposed to certain diseases or disorders.

­SNP testing is the technique used by almost all at-home genetic testing companies. It doesn't, however, provide absolute, undisputed results.

From Spit to SNP: The Basic Process

­Unlik­e at-home pregnancy testing, which delivers results to the user as she watches, at-home genetic testing isn't so simple or home-based. You do get to provide the sample at home, but everything else requires the help of off-site trained scientists. You can't simply spit in a cup, dip in a wand and read the results. This is how it works:

Visit the Web site of your preferred service provider. Three popular services are 23andMe, Navigenics and deCODEme. Next, open an account and order a test. Prices can range from $100 to $2,500, depending on the package you select.

After your order is processed, the company mails a kit to you that includes any necessary equipment.

Now comes the fun part. Using the supplied cup or tube, start collecting your spit. About 30 milliliters (2 tablespoons) of saliva are required to get a sufficient number of cheek cells. The deCODEme service actually uses a buccal DNA collector, which is a stick with rough paper on one end. You rub the paper on the inside of your cheek to collect the cells.

Seal up your sample and place it in the conveniently provided preaddressed envelope.

Mail it and wait patiently.

The lab extracts DNA from your cheek cells and conducts SNP testing to see if you have any markers for certain diseases or disorders.

When your results are ready, usually in about eight to 10 weeks, they're uploaded to your account and you're alerted by e-mail that the data is ready to be reviewed.

What happens next depends on the service provider. Navigenics makes genetic counselors available to help you understand and interpret the data. Social networking is a major goal of the 23andMe service. You can use the company's site to network with other individuals who might share similar backgrounds or proclivities, like wet earwax. Seriously.

­All of the service providers offer security measures to protect your data and allow you to choose how much data is made available to you.

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BYOS: Bring Your Own Spit

If you're not self-conscious about spitting in front of others, you might consider attending or hosting a spit party. The concept comes courtesy of 23andMe, which has hosted several such parties. A typical saliva soirée has the usual mingling and finger food. But it also provides an opportunity to sign up for the testing service. After signing up, guests receive a sample collection kit, retire to a comfortable location and start coughing it up.

Genetic Markers

Different service providers identify different genetic markers.

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­­­So what, exactly, do a few milliliters of spit tell you? The most important thing you'll learn is what kind of genetic markers you carry.

A genetic marker is any alteration in your DNA that may indicate an increased risk of developing a specific disea­se or disorder. Because SNPs are, by their very definition, variations in DNA, they can be used as flags or markers for nearby DNA that affects your health.

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Different se­rvice providers identify different genetic markers. For example, 23andMe identifies your risk for developing lupus, while deCODEme and Navigenics don't. These latter companies, however, test for markers of Alzheimer's disease, which isn't included in 23andMe's repertoire. The table below compares the service providers across 10 other important diseases and disorders [source: Genetics and Public Policy Center].

Understanding your susceptibility to certain diseases is a powerful tool. Say, for instance, your at-home genetic test reveals you're predisposed to develop osteoporosis. You can use this information to take a more proactive role in your own health care. You might decide to take supplements to ensure you're getting enough calcium and vitamin D. You might also engage in regular weight-bearing exercise and opt to have a bone density test to determine your risk for future fracture. These kinds of decisions are part of a growing trend that finds more patients -- well educated and computer-literate -- assuming control of their own health care.

­Better health is not the only thing you can get out of your spit. You can also trace your ancestral roots. This is possible because closely related individuals have more similarities in their DNA. By comparing your genetic information to that of people from around the world, you can fill out a comprehensive family tree, tracing your lineage through either your mother or your father.

Of Maps and Men

The genetics tests we've described so far only analyze bits and pieces of a person's DNA. Given the success of the Human Genome Project, why not sequence an individual's entire genome? The answer comes down to time and money. It takes too long and costs too much to map out all of the 6 billion nucleotides in a person's genome [source: NOVA scienceNOW]. Scientists hope that one day this will be possible. Until then, we'll have to settle for the abridged version of our genetic story.

Genetic Testing Laws

Monsignor Rino Fisichella, who heads the Pontifical Academy for Life, gestures during a press conference at the Vatican on Feb. 17, 2009. The Vatican is warning of the "relentless'' spread of a eugenics mentality with the advance of genetic testing.

­Some people char­acterize at-home genetic testing as a revolution in health care. Others are more cautious. A few are deeply troubled. Much of the concern centers on the uncertainty of results that come back from the testing companies.

Having a genetic marker for a disease is not a guarantee that a person will actually get that disease. Few conditions are purely genetic. Most have both inherited and environmental causes. Many cancers, for example, require a genetic predisposition, as well as an environmental trigger to set the disease in motion. As a result, consumers who make decisions based solely on the results of a genetic test may receive unnecessary treatment or make unnecessary lifestyle changes.

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A few states require consumers to obtain authorization from a health care provider before they undergo genetic testing. Some states prohibit at-home testing altogether. Currently, 25 states and the District of Columbia permit at-home genetic testing without restriction [source: Genetics and Public Policy Center]. The U.S. Food and Drug Administration doesn't regulate direct-to-consumer tests, although the labs themselves must comply with the Clinical Laboratory Improvement Amendments (CLIA), which ensure quality laboratory testing. CLIA doesn't, however, require genetic testing labs to enroll in programs designed to assess their analytic validity -- their ability to deliver correct answers when performing tests.

Finally, there's the issue of genetic discrimination. Many people worry that their genetic information will interfere with their ability to get insurance coverage. In May 2008, the Bush administration took one major step to prevent this by passing the Genetic Information Nondiscrimination Act, or GINA. GINA protects Americans by making it illegal to use genetic information to influence health insurance and employment decisions. It does not extend to genetic information-based discrimination in life or long-term care insurance.

So before you spit into a cup and send it off for testing, you had better make sure you're fully prepared for all consequences, good and bad. Perhaps it's best to remember an old proverb that warns, "Don't spit into the well -- you might drink from it later."